1. Field of the Invention
The present invention generally relates to photoelectric devices, particularly gallium arsenide (GaAs) photovoltaic cells, and more specifically to a photosensitive cell including an environmentally sealed ohmic contact grid interface.
2. Description of the Related Art
Photovoltaic or solar cells including GaAs photosensitive semiconductor layer structures formed on germanium (Ge) substrates are preferred over silicon (Si) based cells due to their higher efficiency in converting light to electricity. A GaAs/Ge solar cell generally includes an aluminum gallium arsenide (AlGaAs) passivation or "window" layer formed over the photosensitive layer structure, and an oxide antireflection (AR) coating formed over the window layer.
GaAs/Ge solar cells are generally classified into two types according to the configuration of their ohmic metal front contact collection grids. The most common is referred to as an etch-through contact, in which front contact patterns are etched through the window layer to expose the underlying GaAs emitter layer of the photosensitive layer structure. Front metals are evaporated and plated directly onto the emitter layer through the etched openings to form the front contact grid. This arrangement is disadvantageous in that, upon exposure to high temperatures such as those experienced in welding, the contact metal can diffuse into the junction of the cell and degrade the performance.
The second type of contact configuration utilizes a cap layer of highly doped, and therefore electrically conductive GaAs to separate the contact metal from the junction of the cell. The cap layer is formed over the entire window layer. A silicon dioxide mask is then formed over the cap layer except in the areas of the contact grid lines. The contact metal is then plated onto the unmasked portions of the cap layer. The silicon dioxide mask is removed, and the cap layer is etched away to expose the window layer except under the contact grid lines. The AR coating is then deposited over the window layer.
The cap layer provides low contact resistance and physically separates the contact metal from the cell junction. The conventional method of fabricating a solar cell including a cap layer is described in an article entitled "26% EFFICIENT MAGNESIUM-DOPED AlGaAs/GaAs CONCENTRATOR SOLAR CELLS" by H Hamaker et all, in Proceedings of 18th Photovoltaic Specialists Conference, IEEE 1985, pp. 327-331. First, a front contact was formed, and then the GaAs cap layer was etched to reveal the AlAgAs window layer. Finally, the AR coatings were deposited.
A representative section of a GaAs/Ge solar photocell 10 having a cap layer configuration fabricated using the prior art method described above is illustrated in FIG. 1. The cell 10 includes an N+doped Ge substrate 12, and a photosensitive GaAs P/N junction layer structure 14 grown on the substrate 12. The structure 14 includes an N+ doped GaAs buffer layer 16, an N- doped GaAs base layer 18 and a P- doped GaAs emitter layer 20. An AlGaAs passivation or window layer 22 is deposited on the emitter layer 20. An oxide AR coating 24 is deposited on the window layer 22. An ohmic metal back contact layer 26 is formed on the surface of the substrate 12 opposite the layer structure 14.
The cell 10 further includes a front ohmic contact collection grid including structures 28 which extend perpendicular to the plane of the drawing in FIG. 1. Only one front contact structure 28 which is illustrated includes a GaAs cap layer 30 extending through an opening 32 in the AR coating 24 to contact the window layer 22. An ohmic metal contact 34 is formed on the cap layer 30.
Light incident on the front of the AR coating 24 causes liberation of electron-hole pairs in the photosensitive layer structure 14 due to the photovoltaic effect. The electrons and holes flow out of the layer structure 14 to an external load (not shown) through the cap layer 30 and front contact 34, and the back contact layer 26 respectively.
The front contact 34 includes a thick layer 34a of silver (Ag) which is plated over the cap layer 30 and a thin gold (Au) layer 34b which is plated over the silver layer 34a. The contact 34 is wider than the cap layer 30. More specifically, the contact 34 has overhangs 34c and 34d on the opposite lateral sides thereof which overlie pockets or hollows 32a and 32b defined between the cap layer 30 and the respective inner walls of the opening 32.
Due to the prior art fabrication process of the cell 10 as described above, the overhangs 34c and 34d are created during the plating step which forms the contact 34. More specifically, a silicon dioxide plating mask (not shown) is formed over the cap layer 30 to cover all but the portion illustrated in FIG. 1. The Ag and Au layers 34a and 34b which form the contact 34 are plated not only onto the exposed upper surface of the cap layer 30, but grow laterally onto the adjacent edge portions of the mask.
The hollows 32a and 32b are created by etching the silicon dioxide plating mask and by etching the cap layer 30 external of the contact 34 to expose the window layer 22. The hollows 32a and 32b are shadowed by the overhangs 34c and 34d of the plated front contact 34 during subsequent evaporation of the AR coating 24, such that the coating 24 extends only to the edges of the overhangs 34c and 34d and the hollows 32a and 32b remain. The AlGaAs window layer 22 in the hollows 32a and 32b is not sealed by the AR coating 24.
AlGaAs corrodes upon exposure to moisture and oxygen. The contact grid interface including the contact 34, cap layer 30 and AlGaAs window layer 22 is environmentally sensitive in that the window layer 22 is directly exposed to the environment in the hollows 32a and 32b. Moisture is able to enter the hollows 32a and 32b and corrode the exposed portions of the window layer 22, thereby reducing the photo-generated current of the cell 10 and degrading its electrical performance.
The hollows 32a and 32b are enlarged during the step of etching the GaAs cap layer 30 to the shape illustrated in FIG. 1. The etchant used to etch the GaAs cap layer 30 also reacts with the Ag layer 34a in the front contact 34. The Au layer 34b is formed on the Ag layer 34a of the contact 34 to protect it from being etched. However, the Au layer 34b on the lower surfaces of the overhangs 34c and 34d is non-uniform and much thinner than on the upper surface of the Ag layer 34a due to the nature of the plating process. This enables the etchant to etch the Ag layer 34a under the overhangs 34c and 34d and thereby enlarge the hollows 32a and 32b.